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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
The moon causes tides on Earth. The tidal effects steal angular momentum from Earth, slowing its rate of rotation. Conservation of angular momentum is maintained by transferring it to the moon, which speeds up in its orbit around Earth. An increase in orbital speed causes the moon to move away from earth. The change is slow, the moon moves about 3 cm farther away from Earth each year. A day on Earth was about eight hours a very long time ago, and the moon was much closer, eventually growing to where it is now.
TLDR: conservation of angular momentum.
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